1. Field of the Invention
The present invention relates to a DC power source unit that can selectively charge a battery pack, which is used as a power source for a cordless power tool, and supply DC voltage to the cordless power tool through an adaptor shaped like the battery pack.
2. Description of the Related Art
Cordless power tools are convenient because they can be used anywhere without being restricted by the length of a power cable. However, whenever the battery runs low, the battery pack needs to be removed and charged or else replaced with another battery pack that has already been charged.
A DC power source unit that converts alternating current to direct current can be used along with the battery pack. That is, the DC power source unit can be used when the cordless power tool is to be operated in a single place that has a source of alternating current, and the battery pack can be used when the cordless power tool is to be operated at several different places that have no source of alternating current. However, this situation has a problem in that the operator of the cordless power tool must carry both the DC power source unit and the battery pack's charging unit to the work area.
Japanese Patent Application Publication No. HEI-5-56566 discloses a DC power source unit with a battery charging function. When the DC power source unit detects that the connected power tool is being operated, it supplies power not only to the cordless power tool but also to the battery pack for charging purposes. On the other hand, when the DC power source unit detects that the power tool is not being operated, then it charges a battery pack.
The DC power source unit of the type disclosed in Japanese Patent Application Publication No. HEI-5-56566 is required to immediately supply an appropriate power to the power tool when driven. In order to meet the requirement, a switching power source incorporated in the DC power source unit is controlled to output a constant voltage suitable for the power tool. However, there arises a problem such that the battery to be charged by the DC power source unit has a rated voltage that is not the same as the voltage applied to the power tool. If the battery is charged with the same voltage applied to the power tool that is higher than the rated voltage of the battery, a rush current instantaneously flows in a switch interposed between the switching power source and the battery immediately after the switch is closed. This is due to a voltage difference between the output of the switching power source and the battery voltage. The rush current shortens the service life of the switch.